In projection television systems, it is common practice to utilize three cathode ray tubes (CRT's) of different colors, namely, red, blue and green. Utilizing three monochromatic CRT's does not require a color corrected lens for normal usage. Examples of such lenses are shown in U.S. Pat. Nos. 4,300,817, 4,348,081 and 4,526,442.
In practice, the phosphors of the three differently colored CRT's emit polychromatically with the green phosphor having significant side bands in blue and red. This chromatic spread can effect the image quality, particularly in situations where high resolution is of prime concern. Where there is to be a data display or large magnification, this color spread manifests itself as lowered image contrast and visible color fringing.
The degree of color correction required in the lenses for these applications depends on the intended application of the lenses.
In general, for lower resolution systems, such as for the projection of typical broadcast television, good color optical performance out to three cycles per millimeter as measured by the modulation transfer function (MTF) is adequate. In these cases, partial color correction may be adequate. For data display via red, green and blue inputs (RGB), and for high definition television, good performance out to ten cycles per millimeter, as measured by the MTF, may be required, and total color correction then becomes necessary.
The requirement for partial or total color correction always complicates an optical design problem. In projection television, it is of vital concern not to alleviate this difficulty by relaxing important system specification, such as field coverage, lens speed, and relative illumination. Additionally, it is often desirable that the lenses be capable of high performance over a significant range of magnifications. A typical front projection requirement might be from a magnification of 10.times. to 60.times.. This further complicates the optical design.
Accordingly, the present invention provides a new and improved projection lens for a cathode ray tube of high definition while maintaining a wide field angle and large relative aperture. The invention also provides a CRT projection lens that maintains a high level of image quality over a wide range of magnifications, for example, 10.times. to 60.times. or greater.